A major industrial problem involves the development of efficient methods for reducing the concentration of air pollutants, such as carbon monoxide, sulfur oxides and nitrogen oxides in waste gas streams which result from the processing and combustion of sulfur, carbon and nitrogen containing fuels. The discharge of these waste gas streams into the atmosphere is environmentally undesirable at the sulfur oxide, carbon monoxide and nitrogen oxide concentrations that are frequently encountered in conventional operations. The regeneration of cracking catalyst, which has been deactivated by coke deposits in the catalytic cracking of sulfur and nitrogen containing hydrocarbon feedstocks, is a typical example of a process which can result in a waste gas stream containing relatively high levels of carbon monoxide, sulfur and nitrogen oxides.
Catalytic cracking of heavy petroleum fractions is one of the major refining operations employed in the conversion of crude petroleum oils to useful products such as the fuels utilized by internal combustion engines. In fluidized catalytic cracking processes, high molecular weight hydrocarbon liquids and vapors are contacted with hot, finely-divided, solid catalyst particles, either in a fluidized bed reactor or in an elongated transfer line reactor, and maintained at an elevated temperature in a fluidized or dispersed state for a period of time sufficient to effect the desired degree of cracking to lower molecular weight hydrocarbons of the kind typically present in motor gasoline and distillate fuels.
In the catalytic cracking of hydrocarbons, some nonvolatile carbonaceous material or coke is deposited on the catalyst particles. Coke comprises highly condensed aromatic hydrocarbons and generally contains from about 4 to about 10 weight percent hydrogen. When the hydrocarbon feedstock contains organic sulfur and nitrogen compounds, the coke also contains sulfur and nitrogen. As coke accumulates on the cracking catalyst, the activity of the catalyst for cracking and the selectivity of the catalyst for producing gasoline blending stocks diminishes. Catalyst which has become substantially deactivated through the deposit of coke is continuously withdrawn from the reaction zone. This deactivated catalyst is conveyed to a stripping zone where volatile deposits are removed with an inert gas at elevated temperatures. The catalyst particles are then reactivated to essentially their original capabilities by substantial removal of the coke deposits in a suitable regeneration process. Regenerated catalyst is then continuously returned to the reaction zone to repeat the cycle.
Catalyst regeneration is accomplished by burning the coke deposits from the catalyst surfaces with an oxygen containing gas such as air. The combustion of these coke deposits can be regarded, in a simplified manner, as the oxidation of carbon and the products are carbon monoxide and carbon dioxide.
When sulfur and nitrogen containing feedstocks are utilized in catalytic cracking process, the coke deposited on the catalyst contains sulfur and nitrogen. During regeneration of coked deactivated catalyst, the coke is burned from the catalyst surface that then results in the conversion of sulfur to sulfur oxides and nitrogen to nitrogen oxides.
The conditions experienced by the catalyst in a fluid catalytic cracking (FCC) unit are very severe. Catalyst is continuously being cycled between reductive atmosphere on the reactor side to an oxidative atmosphere on the regenerator side. The temperatures between the two zones are different so the catalyst experiences thermal shocks. Also the regenerator contains nominally about 15-25% steam. All these factors lead to a significant decline in the catalyst activity and fresh catalyst needs to be continuously added to maintain the cracking activity.
Various approaches have been used to either reduce the formation of noxious gases or treat them after they are formed. Most typically, additives have been used either as an integral part of the FCC catalyst particles or as separate particles in admixture with the FCC catalyst.
The additive that has gained the widest acceptance for lowering sulfur oxide emissions to date in FCC units (FCCU) is based upon Magnesium oxide/Magnesium aluminate/ceria technology. Pt supported on clay or alumina is most commonly used as an additive for lowering of carbon monoxide emissions. Unfortunately the additives used to control CO emissions typically cause a dramatic increase (e.g.  greater than 300%) in NOx evolution from the regenerator.
Various approaches have been used to treat nitric oxide gases in FCCU. For example, U.S. Pat. No. 5,037,538 describes the reduction of oxides of nitrogen (NOx) emissions from an FCC regenerator by adding a deNOx catalyst to the FCC regenerator in a form whereby the deNOx catalyst remains segregated within the FCC regenerator.
U.S. Pat. No. 5,085,762 describes the reduction of emissions of noxious nitrogen oxides with the flue gas from the regenerator of a fluid catalytic cracking plant by incorporating into the circulating inventory of cracking catalyst separate additive particles that contain a copper-loaded zeolite material having a characteristic structure with a defined X-ray diffraction pattern.
U.S. Pat. No. 5,002,654 describes a process for regeneration of cracking catalyst while minimizing NOx emissions using a zinc-based deNOx catalyst.
U.S. Pat. No. 5,021,146 describes a process for regeneration of cracking catalyst while minimizing NOx emissions using a Group IIIb based deNOx additive.
U.S. Pat. Nos. 5,364,517 and 5,364,517 describe the reduction of the NOx content of FCC regenerator flue gas is reduced using a spinel/perovskite additive.
U.S. Pat. Nos. 5,750,020 and 5,591,418 describe process for removing sulfur oxides or nitrogen oxides from a gaseous mixture in an FCC process using a collapsed composition which is substantially composed of microcrystallites collectively of the formula:
M2m2+Al2-pMp3+TrO7+rxc2x7s
where M2+ is a divalent metal, M3+ is a trivalent metal, and T is vanadium, tungsten, or molybdenum.
U.S. Pat. No. 6,165,933 describes compositions comprising a component containing (i) an acidic oxide support, (ii) an alkali metal and/or alkaline earth metal or mixtures thereof, (iii) a transition metal oxide having oxygen storage capability, and (iv) palladium; to promote CO combustion in FCC processes while minimizing the formation of NOx.
U.S. Pat. Nos. 6,129,834 and 6,143,167 describe compositions comprising a component containing (i) an acidic oxide support, (ii) an alkali metal and/or alkaline earth metal or mixtures thereof, (iii) a transition metal oxide having oxygen storage capability, and (iv) a transition metal selected from Groups Ib and/or IIb of the Periodic Table; to provide NOx control performance in FCC processes.
All the additives added to FCC units need to have sufficient hydrothermal stability to withstand the severe environment of an FCCU and there remains the need for NOx additives to be used in FCC that have improved hydrothermal stability.
The invention provides novel compositions suitable for use in FCC processes that are capable of providing improved NOx control performance.
In one aspect, the invention provides compositions for reducing NOx emissions in FCC processes, the compositions containing (i) an acidic oxide support, (ii) ceria (iii) at least one oxide of a lanthanide series element other than ceria, and (iv), optionally, at least one oxide of a transition metal selected from Groups Ib and IIb of the Periodic Table. The acidic oxide support preferably contains alumina. Praseodymium oxide is the preferred lanthanide oxide other than ceria. Cu and Ag are preferred Group Ib transition metals and Zn is the preferred Group IIb transition metal.
In another aspect, the invention encompasses FCC processes using the NOx reduction compositions of this invention either as an integral part of the FCC catalyst particles or as separate particles admixed with the FCC catalyst.
These and other aspects of the invention are described in further detail below.
The invention encompasses the discovery that certain classes of compositions are very effective for the reduction of NOx gas emissions in FCC processes. Moreover, such compositions have unexpectedly improved hydrothermal stability over prior art compositions. The NOx reduction compositions of the inventions are characterized in that they comprise (i) an acidic oxide support, (ii) cerium oxide (iii) at least one oxide of a lanthanide series element other than ceria, and (iv), optionally, at least one oxide of a transition metal selected from Groups Ib and IIb of the Periodic Table and mixtures thereof.
The acidic oxide support should be of sufficient acidity for the composition to act as an effective NOx reduction additive. Acidic oxide catalyst supports are well know to those of ordinary skill in the art and include, for example, transitional aluminas such as gamma and eta alumina, silica-stabilized versions of said aluminas, including the silica-stabilized alumina spinel formed by leaching silica from kaolin calcined through its characteristic exotherm to form the spinel, or mullite. The support may be crystalline or amorphous. Preferably, the acidic oxide support contains at least some alumina. More preferably, the oxide support contains at least 50 wt. % alumina. The oxide support is preferably an oxide selected from the group consisting of alumina and silica-alumina. Where an amorphous silica-alumina support is used, the support preferably has an alumina to silica molar ratio of from about 1:1 up to about 50:1. Examples of commercially available acidic oxide alumina supports are available under tradenames such as PURALOX, CATAPAL and VERSAL. Examples of commercially available acidic silica-alumina supports are available under the tradenames such as SIRAL and SIRALOX.
The silica-alumina support can optionally be created by the caustic leaching of silica from preformed kaolin microspheres as described in U.S. Pat. Nos. 4,847,225 and 4,628,042, which are hereby incorporated by reference for their teachings in this regard. Preferably, the kaolin that is subject to caustic leaching is calcined substantially through its characteristic exotherm to form spinel and/or mullite. More preferably, the caustic leached kaolin support is a microsphere whereby the caustic leached kaolin is bound with aluminum chlorohydroxide before calcination through the exotherm.
The acidic oxide support further preferably has sufficient surface area to facilitate the NOx reduction process. Preferably, the oxide support has a surface area of at least about 20 m2/g, more preferably from about 50 up to about 300 m2/g. The acidic oxide support may be a powder which is preferable when used as an integral part of the FCC catalyst or a microsphere or particle, preferably when used as an admixture with FCC catalysts.
The amount of the cerium oxide (ceria) present in the NOx reduction composition may be varied considerably relative to the amount of acidic oxide support. Preferably, the NOx reduction composition contains at least about 0.5 part by weight of cerium oxide per 100 parts by weight of the acidic oxide support material, more preferably from at least about 1 part by weight up to about 25 parts by weight of cerium oxide per 100 parts of the acidic oxide support material.
The lanthanide oxides other than ceria include at least one metal oxide having oxygen storage capability similar to that of ceria. Preferably, the lanthanide oxide other than ceria is praseodymium oxide. The amount of the lanthanide oxide other than ceria present in the NOx reduction composition may be varied considerably relative to the amount of acidic oxide support. Preferably, the NOx reduction composition contains from at least about 0.05 part by weight of oxide per 100 parts by weight of the acidic oxide support material, more preferably from at least about 1 part by weight up to about 25 parts by weight of lanthanide oxide other than ceria mixture per 100 parts of the acidic oxide support material. The amount of ceria to the lanthanide oxides other than ceria present in the NOx reduction composition of this invention can be as high as 500:1 by weight. Exemplified below are NOx reduction compositions containing an acidic oxide support, ceria, and at least one lanthanide oxide other than ceria, wherein the amount of ceria to the amount of the lanthanide oxides other than ceria is in the range of from 1.66:1 to 5:1 by weight.
The Group Ib and/or IIb transition metals may be any metal or combination of metals selected from those groups of the Periodic Table. Preferably, the transition metal is selected from the group consisting of Cu, Ag, Zn and mixtures thereof. The amount of transition metal present is preferably at least about 100 parts by weight (measured as metal oxide) per million parts of the oxide support material, more preferably from about 0.1 up to about 5 parts by weight per 100 parts of the oxide support material.
The NOx reduction composition may contain minor amounts of other materials, which preferably do not adversely affect the NOx reduction function in a significant way. The NOx reduction composition may consist essentially of items (i)-(iv) mentioned above. Where the composition of the invention is used as an additive particle for an FCC process, the NOx reduction composition may be combined with fillers (e.g. clay, silica-alumina, silica and/or alumina particles) and/or binders (e.g. silica sol, alumina sol, silica alumina sol, etc.) to form particles suitable for use in an FCC process, preferably by spray drying before the calcination of step. More preferably, porous particles, also known as microspheres, are formed from acidic oxide support typically by spray drying powdered oxide support material combined with a binder/filler before or after impregnation with the individual constituents. Preferably, any added binders or fillers used do not significantly adversely affect the performance of the NOx reduction component.
Where the NOx reduction composition is used as an additive particulate (as opposed to being integrated into the FCC catalyst particles themselves), the amount of NOx reduction component in the additive particles is preferably at least 50 wt %, more preferably at least 75 wt. %. Most preferably, the additive particles consist entirely of the NOx reduction component. The additive particles are preferably of a size suitable for circulation with the catalyst inventory in an FCC process. The additive particles preferably have an average particle size of about 20-200 xcexcm. The additive particles preferably have attrition characteristics such that they can withstand the severe environment of an FCCU.
As previously mentioned the NOx reduction composition of the invention may be integrated into the FCC catalyst particles themselves. In such case, any conventional FCC catalyst particle components may be used in combination with the NOx reduction composition of the invention. If integrated into the FCC catalyst particles the NOx reduction composition of the invention preferably represents at least about 0.02 wt. % the FCC catalyst particle, more preferably about 0.1-10 wt. %.
While the invention is not limited to any particular method of manufacture, the NOx reduction composition of the invention is preferably made by the following procedures:
(a) co-impregnate the acidic oxide support particles with a cerium oxide source, at least one lanthanide oxide source other than ceria, and, optionally, at least one source of a Group Ib/IIb element.
(b) calcine the impregnated support of step (a).
The sources of oxides are preferably slurries, sols and/or solutions of the metal oxides themselves or salts of the respective metals, which decompose to oxides on calcination, or combinations of oxides and salts. If desired, the individual constituents may be separately added to the support particles with a calcination step in between each addition. The calcination steps are preferably performed at about 450-750xc2x0 C.
The NOx reduction composition may be used as a separate additive particle or as an integral part of an FCC catalyst particle. If used as an additive, the NOx reduction component may itself be formed into particles suitable for use in a FCC process. Alternatively, the NOx reduction component may be combined with binders, fillers, etc. by any conventional technique. See for example, the process described in U.S. Pat. No. 5,194,413, the disclosure of which is incorporated herein by reference.
Where the NOx reduction component of the invention is integrated into an FCC catalyst particle, preferably the component is first formed and then combined with the other constituents which make up the FCC catalyst particle. Incorporation of the NOx reduction composition directly into FCC catalyst particles may be accomplished by any known technique. Examples of suitable techniques for this purpose are disclosed in U.S. Pat. Nos. 3,957,689; 4,499,197; 4,542,118 and 4,458,023, the disclosures of which are incorporated herein by reference.
The compositions of the invention may be used in any conventional FCC process. Typical FCC processes are conducted at reaction temperatures of 450 to 650xc2x0 C. with catalyst regeneration temperatures of 600 to 850xc2x0 C. The compositions of the invention may be used in FCC processing of any typical hydrocarbon feedstocks. Preferably, the compositions of the invention are used in FCC processes involving the cracking of hydrocarbon feedstocks which contain above average amounts of nitrogen, especially residual feedstocks or feedstocks having a nitrogen content of at least 0.1 wt. %. The amount of the NOx reduction component of the invention used may vary depending on the specific FCC process. Preferably, the amount of NOx reduction component used (in the circulating inventory) is about 0.1-15 wt. % based on the weight of the FCC catalyst in the circulating catalyst inventory. The presence of the compositions of the invention during the FCC process catalyst regeneration step dramatically reduces the level of NOx emitted during regeneration while having improved hydrothermal stability.